This invention relates to drill bits and methods of fabrication, and more particularly to drill bits having a hard abrasion and erosion resistant face and having cutters used in the rotary drilling of bore holes in earth formations.
Typically, earth boring drill bits include an integral bit body which may be of steel or may be fabricated of a hard matrix material such as tungsten carbide. A plurality of diamond or other "superhard" material cutting elements are mounted along the exterior face of the bit body. Each diamond cutting element typically has a backing portion which is mounted in a recess in the exterior face of the bit body. Depending upon the design of the bit body and the type of diamonds used (i.e., either natural or synthetic), the cutters are either positioned in a mold prior to formation of the bit body or are secured to the bit body after fabrication.
The cutting elements are positioned along the leading edges of the bit body so that as the bit body is rotated in its intended direction of use, the cutting elements engage and drill the earth formation. In use, tremendous forces are exerted on the cutting elements, particularly in the forward to rear tangential direction as the bit rotates, and in the axial direction of the bit. Additionally, the bit body and cutting elements are subjected to substantial abrasive and erosive forces.
Typically, the rotary bit includes a fluid flow passage through the interior of the bit which splits into a plurality of passages which are directed to the exterior surface of the bit. These passages, and the exit ports from which fluid is ejected are positioned about the exterior surface of the bit and are directed to impinge high velocity drilling fluid against or across the cutting elements to cool and clean them and to remove adhering cuttings therefrom. The fluid also aids in washing the cuttings from the earth formation upwardly to and through so-called junk slots in the bit to the surface. Again, the high velocity flow of drilling fluid in combination with the cuttings exert tremendous erosive forces on the exterior surfaces of the bit, which also experiences abrasion from contact with the formation being drilled.
Steel body bits have been used for certain earth formations because of their toughness and ductility properties. These properties render them resistant to cracking and failure due to the impact forces generated during drilling. However, steel is subject during drilling operations to rapid erosion from high velocity drilling fluids, and to abrasion from the formation. Typically, such steel body bits have been coated with a hard material such as tungsten carbide to improve erosion resistance. However, tungsten carbide and other erosion resistant materials tend to be brittle. Moreover, there may be thermal expansion mismatches which occur between the steel body and harder material during heat processing which can weaken the bond between the two. During use, the relatively thin coatings may tend to crack and peel, revealing the softer steel body which is then rapidly eroded and abraded. This leads to diamond cutter loss, as the area of the bit supporting the cutter is cut out, and eventual failure of the bit.
Tungsten carbide or other hard metal matrix bits have the advantage of high erosion and abrasion resistance. The matrix bit is generally formed by packing a graphite mold with tungsten carbide powder and then infiltrating the powder with a molten copper alloy binder. A steel blank is positioned in the mold and becomes secured to the matrix as the bit cools after furnacing. Also present in the mold is a mandrel which, when removed after furnacing, leaves behind the fluid passages through the bit. After molding and furnacing of the bit, the end of the steel blank can be welded or otherwise secured to an upper threaded body portion of the bit.
Such tungsten carbide or other hard metal matrix bits, however, are brittle and can crack upon being subjected to impact forces encountered during drilling. Additionally, thermal stresses from the heat applied during fabrication of the bit or during drilling may cause cracks to form. Finally, tungsten carbide and other erosion resistant materials are very expensive in comparison with steel as a material of fabrication.
The problem of fabricating a drill bit which has the desirable properties of toughness and ductility of a steel bit in combination with the erosion resistance of a hard metal matrix bit have been addressed in U. S. application Ser. No. 107,945, filed Oct. 13, 1987, and entitled EARTH BORING DRILL BIT WITH MATRIX DISPLACING MATERIAL. There, a rotary bit is fabricated using a hard metal matrix material which contains a displacement material such as steel powder or steel shot. The displacement material advantageously improves the toughness and ductility of the bit while displacing some of the more expensive hard metal matrix material with a less expensive material.
However, it has been found that bits produced with such displacement material are more subject to erosive and abrasive forces because of the presence of some portion of the displacement material at the exterior face of the bit. Accordingly, there is still a need in the art for a drill bit which has the toughness, ductility, and impact resistance of steel and the hardness as well as abrasion and erosion resistance of tungsten carbide or other hard metal material.